changelog: Add dex client entry

CHANGELOG.md

@@ -11,6 +11,8 @@ incremented for features.
 
 ## [Unreleased]
 
+* spl: Add serum DEX CPI client ([#224](https://github.com/project-serum/anchor/pull/224)).
+
 ## [0.4.4] - 2021-04-18
 
 ## Features